Do all operations using single sed

Issue #980
Signed-off-by: Michal Čihař <michal@cihar.com>

openshift/install.sh

@@ -54,9 +54,9 @@ cd ${OPENSHIFT_REPO_DIR}
 # Pin Django version to 1.8 to avoid surprises when 1.9 comes out.
 # Prevent lxml 3.5 or later to be used on OpenShift because its compilation
 # needs more memory than small gears can provide.
-cat $OPENSHIFT_REPO_DIR/requirements.txt |
-  sed -e 's/Django[<>=]\+.*/Django>=1.8,<1.9/' |
-  sed -e 's/lxml[<>=]\+.*/\0,<3.5/' \
+sed -e 's/Django[<>=]\+.*/Django>=1.8,<1.9/' \
+  -e 's/lxml[<>=]\+.*/\0,<3.5/' \
+  $OPENSHIFT_REPO_DIR/requirements.txt \
   >/tmp/requirements.txt
 
 sh "pip install -U -r /tmp/requirements.txt"