travis: Reduce reruns to 2 and early failure on test failure

While continuing to test all tests is good, it also means we potentially wait
much longer to get the final verdict, and it can truncate logs if there are
too many failures.

.travis/build.sh

@@ -56,7 +56,7 @@ if [ "$NO_PYTHON" != 1 ]; then
 
    cat > pytest.ini << EOF
 [pytest]
-addopts=-p no:logging --color=no --reruns=5
+addopts=-p no:logging --color=no --reruns=2 -x
 EOF
 fi