Add --gcov-options '\-lp' to coverage

Should result in more accurate header file coverage, see
https://github.com/eddyxu/cpp-coveralls/issues/54

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
diff --git a/.travis.yml b/.travis.yml
index 8e98850..cd8589f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -114,7 +114,7 @@
 
 after_success:
     - if [ -n "$COVERALLS" ]; then
-          coveralls -b .;
+          coveralls -b . --gcov-options '\-lp';
       fi;
 
 notifications: