Since GraphX is no longer alpha as of 1.2, MimaExcludes should not exclude GraphX for 1.3

Here are the individual excludes I had to add + the associated commits:

```
            // SPARK-4444
            ProblemFilters.exclude[IncompatibleResultTypeProblem](
              "org.apache.spark.graphx.EdgeRDD.fromEdges"),
            ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.graphx.EdgeRDD.filter"),
            ProblemFilters.exclude[IncompatibleResultTypeProblem](
              "org.apache.spark.graphx.impl.EdgeRDDImpl.filter"),
```
[https://github.com/apache/spark/commit/9ac2bb18ede2e9f73c255fa33445af89aaf8a000]

```
            // SPARK-3623
            ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.graphx.Graph.checkpoint")
```
[https://github.com/apache/spark/commit/e895e0cbecbbec1b412ff21321e57826d2d0a982]

```
            // SPARK-4620
            ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.graphx.Graph.unpersist"),
```
[https://github.com/apache/spark/commit/8817fc7fe8785d7b11138ca744f22f7e70f1f0a0]

CC: rxin

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #3856 from jkbradley/graphx-mima and squashes the following commits:

1eea2f6 [Joseph K. Bradley] moved cleanup to run-tests
527ccd9 [Joseph K. Bradley] fixed jenkins script to remove ivy2 cache
802e252 [Joseph K. Bradley] Removed GraphX MIMA excludes and added line to clear spark from .m2 dir before Jenkins tests.  This may not work yet...
30f8bb4 [Joseph K. Bradley] added individual mima excludes for graphx
a3fea42 [Joseph K. Bradley] removed graphx mima exclude for 1.3