Change travis.yml to provide lxml dependencies

.travis.yml

@@ -53,6 +53,16 @@ addons:
     - python3-mock
     - python3-pyicu
     - python3-coverage
+    # lxml dependencies.  for merge_ref_test to work
+    # - libxml2-dev
+    # - libxslt1-dev
+    - libxml2-utils
+    - python3-lxml
+    - python-libxml2
+# ubuntu 14.04 requires this in addition to libxml2-dev and
+# libxslt1-dev for compiling lxml.
+# https://github.com/deanmalmgren/textract/issues/19
+    - zlib1g-dev
 
 before_install: