Added missing script to build on osx.

_travis/build_on_osx.sh

+#!/bin/bash -
+#===============================================================================
+#
+#          FILE: build_on_osx.sh
+#
+#         USAGE: ./build_on_osx.sh
+#
+#   DESCRIPTION: 
+#
+#       OPTIONS: ---
+#  REQUIREMENTS: ---
+#          BUGS: ---
+#         NOTES: ---
+#        AUTHOR: Dilawar Singh (), dilawars@ncbs.res.in
+#  ORGANIZATION: NCBS Bangalore
+#       CREATED: Tuesday 27 June 2017 01:55:11  IST
+#      REVISION:  ---
+#===============================================================================
+
+set -o nounset                              # Treat unset variables as an error
+set -e
+(
+    brew audit --strict --online moose.rb
+    brew install -V --build-from-source moose.rb
+)