bump the MSRV to 1.40

rrbutani · rrbutani · commit 6787af73a44f · 2020-06-29T22:16:16.000-07:00
rust-lang/rust#63803 we'll get there, eventually
diff --git a/.travis.yml b/.travis.yml
@@ -5,7 +5,7 @@ rust:
   - stable
   - beta
   - nightly
-  - 1.38.0
+  - 1.40.0
 
 install:
   - rustup component add rustfmt-preview clippy-preview || true
