Delete .git source when running the build

This will help us catch bugs that only show up when
we run tests outside of a git checkout.

Author: José Valim

.travis.yml

@@ -1,5 +1,5 @@
 language: erlang
-script: "make compile && make test"
+script: "make compile && rm -rf .git && make test"
 notifications:
   irc: "irc.freenode.org#elixir-lang"
   recipients: