Simplify TravisCI script, which makes travis finish faster

I test installation with the C files that *I* create using this branch:

https://github.com/eriknw/cytoolz/tree/check_my_c_files

Hence, we don't need to verify that cytoolz can be installed with only a
C compiler for every commit.
Merge pull request #35 from eriknw/simplify_travis

Simplify TravisCI script, which makes travis finish faster