Reword this warning to be clearer about what it means
Don't warn when installing externally hosted files

Users who opted to install externally hosted files were being warned
about the possiblility of the availability of those files would be
unreliable. However this has caused some contention and confusion
and at this point they've already decided to do it, so there's no
reason to tell them again.
Remove OSX from our build matrix

The Travis OSX builders are incredibly inconsistent for how long
they take to run our test suite. It is currently ranging anywhere
from ~15 minutes to well beyond 50 minutes, at which point it
times out.
Merge pull request #1807 from dstufft/remove-osx

Remove OSX from our build matrix
Bundle CacheControl and lockfile
Use CacheControl instead of custom cache code

* Deprecates the --download-cache option & removes the download
  cache code.
* Removes the in memory page cache on the index
* Uses CacheControl to cache all cacheable HTTP requests to the
  filesystem.
  * Properly handles CacheControl headers for unconditional
    caching.
  * Will use ETag and Last-Modified headers to attempt to do a
    conditional HTTP request to speed up cache misses and turn
    them into cache hits.
  * Removes some concurrency unsafe code in the download cache
    accesses.
* Adds pip.appdirs to handle platform specific application
  directories such as cache, config, data, etc.