Fixed wrong expression

This should fix leaked coverage from pytest builds

Author: Kojoley

.travis.yml

@@ -230,6 +230,6 @@ after_success:
     else
       echo "Will only deploy docs build from matplotlib master branch"
     fi
-    if [[ $NOSE_ARGS="--with-coverage" ]]; then
+    if [[ $NOSE_ARGS =~ "--with-coverage" ]]; then
       coveralls
     fi