fix infinite loop for resolvable conflicts.

The dependency resolution might enter in an infinite loop if there are
dependency conflicts, even though they can be resolved.
The issue arises because a package causing the dependency conflict is
not removed from the list of unsatisfied names, even if the resolution
is successful, causing it's dependencies to be processed over and over.

This change can be tested with the following requirements:
```
boto3==1.10.16
s3fs
seaborn
```

Signed-off-by: Stefano Bennati <stefano.bennati@here.com>

src/resolvelib/resolvers.py

@@ -412,6 +412,13 @@ def resolve(self, requirements, max_rounds):
                 # Dead ends everywhere. Give up.
                 if not success:
                     raise ResolutionImpossible(self.state.backtrack_causes)
+                else:
+                    criterion = self.state.criteria[name]
+                    # Put newly-pinned candidate at the end. This is essential because
+                    # backtracking looks at this mapping to get the last pin.
+                    for candidate in criterion.candidates:
+                        self.state.mapping.pop(name, None)
+                        self.state.mapping[name] = candidate
             else:
                 # discard as information sources any invalidated names
                 # (unsatisfied names that were previously satisfied)