Use '$HOME' instead of '~'

Problem before this fix:
The character '~' in environment variable is considered as a literal and
thus is not expanded within shell execution.

By using `$HOME` instead, the variable `$TRAVIS_BUILD_DIR` can be safely
used to extend other variables, such as `PATH`.
Merge pull request #83 from gildegoma/use-dollarhome-instead-of-tilde-expansion

Use '$HOME' instead of tilde ('~') to be able to use $TRAVIS_BUILD_DIR in $PATH