travis: fix macOS 10.12 legacy build

just remove all pre installed formulas, since we don't need the majority
of it. after that install what we need. this also fixes the brew update
of those formulas where the source links were broken like popt.

this also helps when the build times out due to building some formulas
from source that are not dependencies we need.

.travis.yml

@@ -120,6 +120,7 @@ before_install:
   - |
         if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
             if [[ "$TRAVIS_OSX_IMAGE" == "xcode9.2" ]]; then
+                brew remove --force $(brew list) --ignore-dependencies
                 brew update
                 pushd "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core"
                 git checkout 55e02323812604add9a69bab8730319b9255a697