Fix Celery to less than 4.3 as requires Django 1.11

[Ed McDonagh]

• We are currently still on Django 1.8.
• Celery 4.3, released today, requires a supported version of Django, ie 1.11

Propose

'celery>=3.1,<4.3'

[Ed McDonagh]

Fixing Celery version to avoid dependency on Django >1.11. Refs #740

→ <<cset 2d945242feb1>>

[Ed McDonagh]

Adding ref #740 to changes

→ <<cset 4dfaef14134f>>

[Ed McDonagh]

Merged in issue740celeryversion (pull request #295)

Fixes #740

→ <<cset a21f574cd997>>

[Ed McDonagh]

Temporary instruction to allow installs of 0.9.0 until 0.9.1 is released. Refs #740

→ <<cset 0641afed537a>>

[Ed McDonagh]

Temporary instruction in linux quick install to allow installs of 0.9.0 until 0.9.1 is released. Refs #740

→ <<cset a898703333a3>>

[Ed McDonagh]

Accidentally committed the changes direct to the docs branch rather than using a PR. Hopefully I'll get away with it this time!