Remove 0.12 from Travis

[jmuk]

As googleapis/google-cloud-node#1855 (comment)

Node 0.12 has ended its official support at the end of 2016, and gRPC has stopped its distributions of precompiled binary packages for 0.12. This caused the failures on Travis -- because node tries to compile gRPC and its old C++ compiler fails to deal with some language features there.

Instead of adding new C++ compiler, we should be aligned with google-cloud-node repository, that means:

• specify the minimum version in the package.json to be 4
• remove 0.12 from Travis
• however, this does not mean to drop the support of 0.12. For example, we can't use new ES6 features which were added to Node v4.