Run Travis tests with PERL_USE_UNSAFE_INC=0

This makes sure that Test::Harness doesn't set PERL_USE_UNSAFE_INC to 1
and that the tests are actually run without . in @INC.
diff --git a/devel/bin/travis-test.sh b/devel/bin/travis-test.sh
index bb7ce13..0927a29 100755
--- a/devel/bin/travis-test.sh
+++ b/devel/bin/travis-test.sh
@@ -45,6 +45,9 @@
 }
 
 test_perl() {
+    # Test::Harness defaults to PERL_USE_UNSAFE_INC=1
+    export PERL_USE_UNSAFE_INC=0
+
     source ~/perl5/perlbrew/etc/bashrc
     perlbrew switch $PERL_VERSION
     perlbrew list