ARROW-3921: [GLib][CI] Log Homebrew output

Homebrew stuck problem is still occurred: #3095 (comment)

This change starts logging Homebrew output to debug this case.

Author: Kouhei Sutou <kou@clear-code.com>

Closes #3113 from kou/glib-ci-log-homebrew and squashes the following commits:

6085620 <Kouhei Sutou>  Log Homebrew output

.travis.yml

@@ -332,6 +332,10 @@ after_failure:
       fi
       ls -la ~/Library/Logs/DiagnosticReports/
       cat ~/Library/Logs/DiagnosticReports/*.crash
+      if [[ -f brew.log ]]; then
+        echo "Homebrew log"
+        cat brew.log
+      fi
     else
       COREFILE=$(find . -maxdepth 2 -name "core*" | head -n 1)
       if [[ -f "$COREFILE" ]]; then

ci/travis_install_osx.sh

@@ -20,8 +20,14 @@
 set -e
 
 if [ "$ARROW_CI_RUBY_AFFECTED" = "1" ]; then
-    brew update
-    brew upgrade python
-    brew uninstall postgis
-    brew bundle --file=$TRAVIS_BUILD_DIR/c_glib/Brewfile
+    brew_log_path=brew.log
+    function run_brew() {
+        echo brew "$@" >> ${brew_log_path}
+        brew "$@" >> ${brew_log_path} 2>&1
+    }
+    run_brew update
+    run_brew upgrade python
+    run_brew uninstall postgis
+    run_brew bundle --file=$TRAVIS_BUILD_DIR/c_glib/Brewfile --verbose
+    rm ${brew_log_path}
 fi