Pin gnustep to 1.9 to avoid compilation issues

Apparently something added since 1.9 makes clang 5.0 on linux fail an
assertion.

Author: SSheldon

travis_install.sh

@@ -3,7 +3,7 @@
 set -eu
 
 gnustep_install() {
-    git clone https://github.com/gnustep/libobjc2.git
+    git clone -b 1.9 https://github.com/gnustep/libobjc2.git
     mkdir libobjc2/build
     cd libobjc2/build
     export CC="clang"