Provide a default version for leiningen

For odd cases where leiningen is called without the wrapper shell script, any occurence of `:min-lein-version` will throw an exception since `leiningen-version` will return nil.

This fixes jenkinsci/leiningen-plugin#1 for instance.
I see two other ways to fix this:

- have the version be propagated from project.clj directly, through substitution
- let leiningen-version return a string that always passes or always fails (not sure which is best) the `:min-lein-version` requirement
Fetch version from `pom.properties`

After clarifying things with @technomancy, it makes more sense
to pull the version from the already provisionned pom.properties
file.
Merge pull request #1109 from pyr/patch-1

Provide a default version for leiningen
Reorder defs
Extract connect-string function
More reordering + make testable fns public
Fix missing args in connect-string
Tests for repl
Fix #1103 by bumping stencil dependency.
Only try to open file when needed in lein-version.

(This should fix the breaking tests.)
Allow IObjs to be cmp. to non IObjs when merging.

Fixes #1110.
Add tests for IObj vs. non IObjs merging.
GPG doc updates.

This includes:

* publishing to a keyserver
* revocation
* setting the default signing key via lein
Add support for no_proxy environment variable

Closes #816.
Move Tiered Compilation to LEIN_JVM_OPTS and document.

Fixes #1111.
Move default :test-selectors to :leiningen/test profile.

Fixes #1099.
Merge pull request #1114 from joelittlejohn/system-no-proxy

Add support for no_proxy environment variable
Fix self-install dev heuristic to refuse to operate on a checkout.

Fixes #1106.
Use `file.separator` in pom path relativization.

Fixes #944 I think.
Add a note about testing the repl task.