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

Problem before this fix:
Character `~` in environment variable is considered as a literal and
thus is not expanded within shell execution. It is thus not yet possible to use `TRAVIS_BUILD_DIR` to customize the `PATH` environement variable. 
 * Integration problem example: https://travis-ci.org/gildegoma/cappuccino/builds/4877254 
 * Reference: http://stackoverflow.com/questions/3984074/tilde-expansion-in-environment-variable

**Solution:** By using `$HOME` instead, the variable `$TRAVIS_BUILD_DIR` can be safely
used to extend other variables, such as `PATH`.

**Related issue:** By removing these `~`, I also planned to pack in this pull request to replace all `~/.ssh` with a Ruby variable. I already started to work on it, but I'll propose it in the frame of this pull request only if Travis team members confirm the interest to unify the way to refer to "home" resources...