Update the vim package version we use on Travis

Also, drop `--not-a-term` because it's only available in vim 8.

Author: jparise

.travis.yml

@@ -1,3 +1,6 @@
 language: vim
+before_install: |
+  sudo apt-get -qq update
+  sudo apt-get -y install vim
 script:
   - make test

Makefile

@@ -3,7 +3,7 @@
 all: test
 
 test: test/vader.vim
-	cd test && vim -Nu vimrc -c 'Vader! *' --not-a-term > /dev/null
+	cd test && vim -Nu vimrc -c 'Vader! *' > /dev/null
 
 test/vader.vim:
 	git clone --depth 1 https://github.com/junegunn/vader.vim test/vader.vim