drop gcc-4.9 builds from Travis CI
drop gcc-4.9 builds from Travis CI

Summary: gcc-4.9 is old and is no longer supported by folly.

Reviewed By: phoad

Differential Revision: D10408986

fbshipit-source-id: 68aa978f4f27ac1419aac26377433e399243ad37

phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp