Test httplib2 transport package on travis (#135)

diff --git a/httplib2_transport/MANIFEST.in b/httplib2_transport/MANIFEST.in
new file mode 100644
index 0000000..aac7fe1
--- /dev/null
+++ b/httplib2_transport/MANIFEST.in
@@ -0,0 +1,2 @@
+include README.rst LICENSE
+recursive-include tests *
diff --git a/httplib2_transport/tox.ini b/httplib2_transport/tox.ini
index 5d7457e..84635e8 100644
--- a/httplib2_transport/tox.ini
+++ b/httplib2_transport/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = lint,py27,py34,py35,pypy,cover
+envlist = lint,py27,py34,py35,py36,pypy,cover
 
 [testenv]
 deps =
@@ -16,7 +16,7 @@
   py.test --cov=google_auth_httplib2 --cov=tests {posargs:tests}
 
 [testenv:cover]
-basepython = python3.5
+basepython = python3.6
 commands =
   py.test --cov=google_auth_httplib2 --cov=tests --cov-report= tests
   coverage report --show-missing --fail-under=100
diff --git a/scripts/travis.sh b/scripts/travis.sh
index 7359a0c..9e11de8 100755
--- a/scripts/travis.sh
+++ b/scripts/travis.sh
@@ -14,6 +14,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+set -eo pipefail
+
 DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 ROOT=$( dirname "$DIR" )
 
@@ -36,4 +38,12 @@
 fi
 
 # Run tox.
+echo "Running tox..."
 tox
+
+# Run tox for sub-packages.
+if [[ $TOXENV != "docs"  && -z $SYSTEM_TEST ]]; then
+    echo "Running tox for httplib2_transport..."
+    cd httplib2_transport
+    tox
+fi