build, docs: Improve macOS building

- New docker command for quickly running tasks in a Linux environment.
- Updated docs with macOS specific details.
- Fixed some test issues.
- Add (fallible) macOS test target for Travis.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,7 @@
 ## Tips:
 
 * please read the style guide before submitting your patch:
-[docs/style-guide.md](docs/style-guide.md)
+[docs/style-guide.md](../docs/style-guide.md)
 
 * commit message titles must be in the form:
 ```topic: Capitalized message with no trailing period```