Use sudo to fix 'pip install' in travis

.travis.yml

@@ -2,6 +2,9 @@
 # Distributed under the Boost Software License, Version 1.0. (See accompanying
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+sudo: required
+dist: trusty
+
 addons:
     apt:
         packages:
@@ -12,7 +15,7 @@ addons:
 
 before_script:
     - touch Jamroot.jam
-    - pip install lxml
+    - sudo pip install lxml
 
 script:
     - cd ${TRAVIS_BUILD_DIR}/test