Fix the build by merging both db migrations heads (apache#5464)

CONTRIBUTING.md

@@ -505,3 +505,21 @@ https://github.com/apache/incubator-superset/pull/3013
   with a PGP key and providing MD5, Apache voting, as well as
   publishing to Apache's SVN repository. View the ASF docs for more
   information.
+
+
+## Merging DB migrations
+
+When 2 db migrations collide, you'll get an error message like this one:
+
+```
+  alembic.util.exc.CommandError: Multiple head revisions are present for
+  given argument 'head'; please specify a specific target
+  revision, '<branchname>@head' to narrow to a specific head,
+  or 'heads' for all heads`
+```
+
+To fix it, first run `superset db heads`, this should list 2 or more
+migration hashes. Then run
+`superset db merge {PASTE_SHA1_HERE} {PASTE_SHA2_HERE}`. This will create
+a new merge migration. You can then `superset db upgrade` to this new
+checkpoint.

superset/migrations/versions/705732c70154_.py

@@ -0,0 +1,22 @@
+"""empty message
+
+Revision ID: 705732c70154
+Revises: ('4451805bbaa1', '1d9e835a84f9')
+Create Date: 2018-07-22 21:51:19.235558
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '705732c70154'
+down_revision = ('4451805bbaa1', '1d9e835a84f9')
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    pass
+
+
+def downgrade():
+    pass