Cache built dependency packages on Travis CI to speed up testing flow

[yan12125 (Chih-Hsuan Yen)]

Motivation

I just submitted a new port libfilezilla. Builds on Travis CI timed out after 48 minutes [1] while on buildbot it takes about 8 minutes only [2]. I believe what @raimue said [3] - time to build non-redistributable dependencies - is the key difference. I hope we can have a caching mechanism so that newcomers can know if their patches are OK or not quickly, and maintainers can spend less time on testing patches manually.

Proposal

Travis CI provides a simple caching mechannism [4]. If built packages under $PREFIX/var/macports/software are cached, there's no need to rebuild them every time.

Cached data are only accessible to Travis CI builders. There are no URLs to cached files in build logs. I guess it's not a "redistribution" as other people can't access them. Thus there are no licensing issues.

[1] ​https://travis-ci.org/macports/macports-ports/builds/273323633

[2] ​https://build.macports.org/builders/ports-10.12_x86_64-builder/builds/38131

[3] ​https://github.com/macports/macports-ports/pull/756#issuecomment-328345604

[4] ​https://docs.travis-ci.com/user/caching/