Workaround for homebrew bug with osx shard (pantsbuild#7050)

### Problem
Homebrew fails to execute when running `brew upgrade`. This leads to the OSX Rust + Platform Specific Tests shard consistently failing.

### Solution
Use workaround described in Homebrew/brew#5513.

.travis.yml

@@ -558,6 +558,11 @@ matrix:
       osx_image: xcode8.3
       stage: *test
       language: generic
+      # TODO: workaround for Homebrew bug triggered by running `brew update`. 
+      # Follow at https://github.com/Homebrew/brew/issues/5513 and remove this workaround once its fixed.
+      env:
+        - HOMEBREW_LOGS=~/homebrew-logs
+        - HOMEBREW_TEMP=~/homebrew-temp
       before_install:
         - brew tap caskroom/cask && brew update && brew cask install osxfuse
       before_script:

build-support/travis/travis.yml.mustache

@@ -274,6 +274,11 @@ matrix:
       osx_image: xcode8.3
       stage: *test
       language: generic
+      # TODO: workaround for Homebrew bug triggered by running `brew update`. 
+      # Follow at https://github.com/Homebrew/brew/issues/5513 and remove this workaround once its fixed.
+      env:
+        - HOMEBREW_LOGS=~/homebrew-logs
+        - HOMEBREW_TEMP=~/homebrew-temp
       before_install:
         - brew tap caskroom/cask && brew update && brew cask install osxfuse
       before_script: