Try and upgrade zef

Author: JJ

shippable.yml

@@ -4,6 +4,6 @@ build:
     - echo "deb http://dl.bintray.com/nxadm/perl6-doc trusty main" | sudo tee -a /etc/apt/sources.list
     - apt-get update && apt-get install rakudo-pkg-for-doc libtap-formatter-junit-perl
     - PATH="/opt/rakudo-pkg-for-doc/bin:/opt/rakudo-pkg-for-doc/share/perl6/site/bin:$PATH"
-    - zef update && zef install Perl6::TypeGraph
+    - zef upgrade zef && zef update && zef install Perl6::TypeGraph
     - P6_DOC_TEST_VERBOSE=1 prove --timer --formatter=TAP::Formatter::JUnit -e perl6 t > shippable/testresults/doctests.xml