Dockerfile: Explicitly depend on "dockcross/base:latest"

As recommended in by CircleCI, explicitly specify the "latest"
tag. This will be important when we will start versioning our images.

Copied from CircleCI documentation [1]:

"Make sure you tag the image you use in the FROM command in your
Dockerfile, even if it is the default “latest” tag. Otherwise, Docker
will pull down all tags for the image you specify. This will create
significant performance problems because the save/load strategy described
above only caches the image layers (and thus tags) that you specify in
the docker save command, so other tags will be re-pulled on every build
if a tag is not specified in the FROM command."

[1] https://circleci.com/docs/docker/#caching-docker-layers

linux-x64/Dockerfile

@@ -1,4 +1,4 @@
-FROM dockcross/base
+FROM dockcross/base:latest
 MAINTAINER Matt McCormick "matt.mccormick@kitware.com"
 
 ENV CROSS_TRIPLE x86_64-linux-gnu