Add 2.8.0-rc0

Right now, `update.sh` can't handle the RCs because it's based on the Debian repo (since there's not really a better place to glean "latest versions" from), but it will skip them for now (so we'll have to watch for RCs manually and bump as appropriate, which is easy enough by following their blog which is how I noticed 2.8.0-rc0 in the first place).  Once 2.8.0 is officially released, `update.sh` will automatically pick it up and run with it appropriately.

```console
$ ./update.sh
+ '[' -z 2.2.7 ']'
+ sed -ri 's/^(ENV MONGO_VERSION) .*/\1 2.2.7/' 2.2/Dockerfile
+ sed -ri 's/^(RUN gpg .* --recv-keys) [0-9a-fA-F]*$/\1 3AFEF01FE92B6927CC1EEC80F564179A36496327/' 2.2/Dockerfile
+ '[' -z 2.4.12 ']'
+ sed -ri 's/^(ENV MONGO_VERSION) .*/\1 2.4.12/' 2.4/Dockerfile
+ sed -ri 's/^(RUN gpg .* --recv-keys) [0-9a-fA-F]*$/\1 CEA1E18DDA77EF4E67884FF2A6982D0160456C5A/' 2.4/Dockerfile
+ '[' -z 2.6.5 ']'
+ sed -ri 's/^(ENV MONGO_VERSION) .*/\1 2.6.5/' 2.6/Dockerfile
+ sed -ri 's/^(RUN gpg .* --recv-keys) [0-9a-fA-F]*$/\1 DFFA3DCF326E302C4787673A01C4E7FAAAB2461C/' 2.6/Dockerfile
+ '[' -z 2.7.8 ']'
+ sed -ri 's/^(ENV MONGO_VERSION) .*/\1 2.7.8/' 2.7/Dockerfile
+ sed -ri 's/^(RUN gpg .* --recv-keys) [0-9a-fA-F]*$/\1 BDC0DB28022D7DEA1490DC3E7085801C857FD301/' 2.7/Dockerfile
+ '[' -z '' ']'
+ sed -ri 's/^(RUN gpg .* --recv-keys) [0-9a-fA-F]*$/\1 BDC0DB28022D7DEA1490DC3E7085801C857FD301/' 2.8/Dockerfile
```

```console
$ ./generate-stackbrew-library.sh
# maintainer: InfoSiftr <github@infosiftr.com> (@infosiftr)

2.2.7: git://github.com/docker-library/mongo@2881be3e0d85b3600bda8d45050a2ceb3f7bb12b 2.2
2.2: git://github.com/docker-library/mongo@2881be3e0d85b3600bda8d45050a2ceb3f7bb12b 2.2

2.4.12: git://github.com/docker-library/mongo@6fcf234e0bd9b50cfbac267ccb451cb8500c8dfc 2.4
2.4: git://github.com/docker-library/mongo@6fcf234e0bd9b50cfbac267ccb451cb8500c8dfc 2.4

2.6.5: git://github.com/docker-library/mongo@bebabb72a2785d7b78309bf6be6c82bab882574b 2.6
2.6: git://github.com/docker-library/mongo@bebabb72a2785d7b78309bf6be6c82bab882574b 2.6
2: git://github.com/docker-library/mongo@bebabb72a2785d7b78309bf6be6c82bab882574b 2.6
latest: git://github.com/docker-library/mongo@bebabb72a2785d7b78309bf6be6c82bab882574b 2.6

2.7.8: git://github.com/docker-library/mongo@9c3312e0f6cc6118bab840105ce701067a84feff 2.7
2.7: git://github.com/docker-library/mongo@9c3312e0f6cc6118bab840105ce701067a84feff 2.7

2.8.0-rc0: git://github.com/docker-library/mongo@a226987aaeee9f81aee4de5406eb55192aefeaa4 2.8
2.8.0: git://github.com/docker-library/mongo@a226987aaeee9f81aee4de5406eb55192aefeaa4 2.8
2.8: git://github.com/docker-library/mongo@a226987aaeee9f81aee4de5406eb55192aefeaa4 2.8
```