Update superset to 0.34.0

[carlosms]

New stable release: https://github.com/apache/incubator-superset/releases/tag/0.34.0

[smacker]

No need to update to 0.34.0. It has absolutely the same code as rc1 which we are using right now but with updated changelog / some minor license updates.

[carlosms]

Since we are low on tasks, this could be a good chance for someone other than @smacker to get familiar with the process described in https://github.com/src-d/sourced-ui/blob/master/CONTRIBUTING.md#merging-upstream (BTW good job describing the steps @smacker)

[smacker]

rc1 was cut from master which makes it simplier to merge future releases. 0.34.0 was cut from release branch if we merge it, it will require reverts or manual conflict resolution.

[dpordomingo]

As discussed during the last TFM, we'll wait till the next oficial release, because this one does not add much value compared with the current 0.34.0rc1