Travis CI: Exercise tests both GOMAXPROCS=`nproc` and GOMAXPROCS=1

GOMAXPROCS=1 currently triggers tests hang:
https://github.com/hanwen/go-fuse/issues/261

Change-Id: I3dc98c8bbdaac7a4ce86639e405ba1a1cef4c295

.travis.yml

@@ -15,6 +15,11 @@ go:
   - 1.15.x
   - master
 
+env:
+  - GOMAXPROCS=1
+  # vvv = native `nproc`
+  - GOMAXPROCS=
+
 matrix:
  fast_finish: true
  allow_failures: