Add Travis job against the new resolver

.travis.yml

@@ -11,6 +11,9 @@ stages:
 - primary
 - secondary
 
+env:
+  - PIP_UNSTABLE_FEATURE=
+
 jobs:
   include:
     # Basic Checks
@@ -51,7 +54,19 @@ jobs:
     - env: GROUP=2
       python: 3.5
 
+    # Test experimental stuff that are not part of the standard pip usage.
+    # Helpful for developers working on them to see how they're doing.
+    - stage: experimental
+      env:
+        - GROUP=1
+        - PIP_UNSTABLE_FEATURE=resolver
+    - env:
+      - GROUP=2
+      - PIP_UNSTABLE_FEATURE=resolver
+
   fast_finish: true
+  allow_failures:
+    - env: PIP_UNSTABLE_FEATURE=resolver
 
 before_install: tools/travis/setup.sh
 install: travis_retry tools/travis/install.sh