Build conda packages for the master branch

[nudles]

The update frequency of the dev branch, master branch and version is high, middle and low respectively. However, the current CI builds the conda package once the master has new commits. I suggest to

1. build the package when there is a new commit in master branch or a new release. Do not build conda packages when there is a new commit to the dev branch.
2. name the package built against the master branch with the tag 'next' in the build string, which is to be consistent with the documentation site like http://singa.apache.org/docs/next/installation/. But the internal version is still the latest release version. singa-gpu and singa-cpu still points to the latest release versions.