license: fix up MIT license so it's an exact match

Before:

```console
$ licensee diff --license mit LICENSE-MIT
Comparing to MIT License:
Input Length:      1092
License length:    1020
Similarity:      92.46%
diff --git a/LICENSE b/LICENSE
index 0ce42af..be0ff1c 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,3 +1,4 @@
{+spack project developers. see the top-level copyright file for details.+}
permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "software"), to deal in
the software without restriction, including without limitation the rights to
```

After:

```console
$ licensee diff --license mit LICENSE-MIT
Comparing to MIT License:
Input Length:       1020
License length:     1020
Similarity:      100.00%
Exact match!
```

This gets us a 100% license match from GitHub's `licensee` tool.

Author: tgamblin

LICENSE-MIT

@@ -1,20 +1,21 @@
-Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
-Spack Project Developers. See the top-level COPYRIGHT file for details.
+MIT License
 
-Permission is hereby granted, free of charge, to any person obtaining a
-copy of this software and associated documentation files (the "Software"),
-to deal in the Software without restriction, including without limitation
-the rights to use, copy, modify, merge, publish, distribute, sublicense,
-and/or sell copies of the Software, and to permit persons to whom the
-Software is furnished to do so, subject to the following conditions:
+Copyright (c) 2013-2020 LLNS, LLC and other Spack Project Developers.
 
-The above copyright notice and this permission notice shall be included in
-all copies or substantial portions of the Software.
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
 
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
-FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
-DEALINGS IN THE SOFTWARE.
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.