Enable Travis CI test for pip installation on OSX

[danielhers]

#596 enabled Travis CI test for pip installation on Linux, but on OSX it doesn't work because libdynet.dylib is not found. This can be resolved by adding its location to DYLD_LIBRARY_PATH. It's not $TRAVIS_BUILD_DIR/build/dynet because the pip installation creates a build directory inside build called something like py3.4-64bit, but instead of trying to figure that out, it should be possible to use the virtualenv, something like $(dirname $(which python))/../lib because it installs it into that location.

I've been trying forever to make the Python bindings work on OSX without requiring DYLD_LIBRARY_PATH, but I can't figure out how to add the lib properly to RUNTIME_LIBRARY_DIRS. Perhaps someone with more Mac experience can help.

[neubig]

Thanks for the pointer! This would be really nice to fix, so hopefully someone could help. I'll also take a look but I'm a bit swamped at the moment.

[danielhers]

So am I, otherwise I would have done it myself :)
I did open #783, which is related, and took a lot more time than I expected to fix...