Speed up OSX builds on Travis

[forejtv]

Do not autoupdate Homebrew, and limit cache size to 1G.

[tautschnig]

Is there any documentation or other background that you might be able to provide? From the outside, those changes seem perfectly consistent, but also rather random.

[forejtv]

@tautschnig Yes, sure. It turns out ccache on Travis on OSX is not ideally configured, for two reasons.

• It needs to be installed via Homebrew. By default, any install via Homebrew triggers an update, which adds ~1min to the job time. The switch we add disables the upgrade.
• Max cache size is 5GB wich actually makes Travis timeout on cache upload (maximal time for upload is 3mins). As a consequence, compilation does not take advantage of caching (cache is not stored from previous build), plus there is 3 minute delay until the upload fail. See examples when it did not work and an exmple from when the cache was small and it worked. Setting cache to 1GB brings OSX in line with how it's set on Travis's Linux build