⚡ Support linux builds for faster CI.

[imlucas]

cc @kangas

[kangas]

Testing now on Ubuntu 15.04

[imlucas]

@kangas sweet! Having travis be able to verify that everything actually compiles is the huge win here https://magnum.travis-ci.com/10gen/compass. Faster turnaround on evergreen definitely a boost as well. Dropped tickets for getting to official linux support under INT-691.

[kangas]

kangas@beefsupreme:~/workspace/compass (pr/149)
$ npm run ci
...
[11:42:31] Finished 'npm:install' after 1.43 min
[11:42:31] Starting 'build:js'...
[11:42:42] Finished 'build:js' after 11 s
[11:42:42] Finished 'build' after 1.67 min
[11:42:42] Starting 'electron:build'...
Downloading electron-v0.30.6-linux-x64.zip
[============================================>] 100.0% of 35.99 MB (7.58 MB/s)
Packaging app for platform linux x64 using electron v0.30.6
[11:42:55] Finished 'electron:build' after 14 s
[11:42:55] Starting 'electron:build-installer'...
⚠  Linux installers coming soon!
[11:42:55] Finished 'electron:build-installer' after 361 μs
[11:42:55] Finished 'release' after 1.88 min

Wee!

Only hiccup was:

> kerberos@0.0.15 install /data02/workspace/compass/build/node_modules/kerberos
> (node-gyp rebuild) || (exit 0)

make: Entering directory '/data02/workspace/compass/build/node_modules/kerberos/build'
  CXX(target) Release/obj.target/kerberos/lib/kerberos.o
In file included from ../lib/kerberos.cc:1:0:
../lib/kerberos.h:5:27: fatal error: gssapi/gssapi.h: No such file or directory
 #include <gssapi/gssapi.h>
                           ^
compilation terminated.
kerberos.target.mk:94: recipe for target 'Release/obj.target/kerberos/lib/kerberos.o' failed

So we have to remember to install the correct libs & headers, whatever they are.

note @imlucas: Tracked as https://jira.mongodb.org/browse/INT-693