travis: clone master branch only
travis: clone master branch only

It reduces a cost of clone.

Before:

  git clone --depth 1 --recursive git@github.com:groonga/groonga groonga.depth1  6.27s user 2.24s system 16% cpu 52.319 total

After:

  git clone --depth 1 --recursive --branch master git@github.com:groonga/groonga  6.18s user 2.04s system 24% cpu 33.450 total

naoa/groonga,groonga/groonga,naoa/groonga,naoa/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,hiroyuki-sato/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,redfigure/groonga,kenhys/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,komainu8/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,redfigure/groonga,groonga/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,kenhys/groonga,redfigure/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,hiroyuki-sato/groonga,redfigure/groonga,naoa/groonga,naoa/groonga,redfigure/groonga,cosmo0920/groonga,komainu8/groonga,hiroyuki-sato/groonga,redfigure/groonga,komainu8/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,kenhys/groonga