Valid PATH manipulation on macOS

`pip install package-name --user` installs binaries to USER_SITE.
USER_SITE is different on different systems:
https://docs.python.org/3/library/site.html#site.USER_SITE

Using `python -m site --user-base` makes travis-cargo valid command:
https://docs.python.org/3/library/site.html#cmdoption-site--user-site

Author: opilar

.travis.yml

@@ -6,7 +6,7 @@ rust:
 before_script:
 - |
   pip install 'travis-cargo<0.2' --user &&
-  export PATH=$HOME/.local/bin:$PATH
+  export PATH=`python -m site --user-base`/bin:$PATH
 script:
 - |
   travis-cargo build &&