Support multiple ArangoDB versions

[aurelijusb]

Problem

• Various companies use various ArangoDB versions.
• Need to support pre-release versions.
• Other developers create forks just to support newer versions: https://github.com/jaggernoth/scarango
• With more API coverage there will be more compatibility issues

Tasks

• Update tests: to support overwriting ArangoDB version via environment variable
• Update Travis integration: matrix builds
• Compare test speed for ArangoDB as a dependencies via docker versus TravisCI build