Update all dockerfiles for migration to Go 1.4+

[xizhao]

Srclib uses features of go 1.4
Dockerfiles are currently set to ubuntu:14.04
In ubuntu:14.04's registry, go 1.2.1 is the latest release
So go must be installed manually to support newer versions of srclib

RUN wget https://storage.googleapis.com/golang/go1.4.2.linux-amd64.tar.gz
RUN sudo tar -C /usr/local -xzf go1.4.2.linux-amd64.tar.gz
ENV PATH /usr/local/go/bin:${PATH}