Try testing against more than just one version of Eliot.
Merge remote-tracking branch 'origin/master' into travis-eliot-head-config-23
expose the minimum supported version of eliot for the benefit of travis
use the information now exposed by setup.py to get rid of a bunch of gross shell
Try using the block string literal syntax to ensure all of the environment definitions are definitely strings.  Then, also quote the values.

This is intended to fix the problem where travis kerplodes with:

```
$ export ELIOT=eliot==$(python
/home/travis/build.sh: line 158: unexpected EOF while looking for matching `)'
/home/travis/build.sh: line 283: syntax error: unexpected end of file
```
Delay lookup of the eliot version because the source is not yet checked out when the environment is evaluated.
It is necessary to mention which package is to be installed.
Cannot just put `eliot==` there.  It makes the string no longer valid for shell evaluation.
Merge pull request #25 from hybridlogic/travis-eliot-head-config-23

Configure travis to run the tests against the minimum, current, and development head versions of Eliot.