fix(Dockerfile): Use ubuntu:trusty image

Ubuntu 14.04 is stable and has golang packaged natively. This means that
the golang compiler and runtime binaries are signed and trusted by the
Ubuntu team and you do not need to muck around with manually installing
golang. This also means you use the common base of Ubuntu 14.04 and don't
fill people's disks with Debian images [they may not use][LeastSurprise]
(based on download count and tag numbers of `ubuntu` vs `debian`).

Additionally, this should be changed to `deis/base` after deis/base#7 is
merged so that `helloworld` can be more consistent with the other parts
of deis.

[LeastSurprise]: https://en.wikipedia.org/wiki/Principle_of_least_astonishment