travis: use "make -k" to see all failures

One of the criticisms of Travis on the call earlier today was that
when there's a compile failure, you really only see the *first*
compile failure.  Then you fix that, resubmit, and 15-60 minutes
later, you see the *next* compile failure.

Per some discussion with @bosilca and @rhc54 this afternoon, use "make
-k", which will have "make" try to continue the build even after
compile failures.  Hence, you'll be able to see many more compile
errors that just one at a time.

In some cases (e.g., if there's a typo in a top-level header file), it
will cause a bazillion lines of compile error output, but hopefully
that should be easy to spot / ignore.

.travis.yml

@@ -46,7 +46,7 @@ install:
     - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then libtool --version; else glibtool --version; fi
     - ./autogen.pl
     - if [[ "$TRAVIS_OS_NAME" == "linux" && "$CC" == "gcc" ]]; then ./configure $CONFIGURE_ARGS --with-libfabric=$HOME/bogus --with-usnic --with-verbs; else ./configure $CONFIGURE_ARGS; fi
-    - make
+    - make -k
 
 script:
     - if [[ "$TRAVIS_OS_NAME" == "linux" && "$CC" == "gcc" ]]; then make distcheck; else make check; fi