Compatibility with 17.0-rc2

Type queue() warns on 17.0, queue:queue() warns on R16, so disable
warnings_as_errors for now.
Merge pull request #42 from ericmj/compat-17

Compatibility with 17.0-rc2
Update .travis.yml
Bump the version to 1.2.0 and add a Hex package definition
Include version in package.exs

Otherwise, the project is not compiled successfully.

After this change is applied, I would advise for a new
package to be published on hex.pm.
Merge pull request #48 from josevalim/patch-1

Include version in package.exs
Bump version to 1.2.1
Use workers as a stack, not a queue

Instead of round-robin of worker selection from the pool, we now use a
LIFO principle to select a worker.  Rationale behind this is that 'hot'
workers are better workers to use than cold workers.

For instance, in database applications, worker processes might decide to
close their connection after being idle for a while (see devinus'
comment on https://github.com/devinus/poolboy/issues/30). Doing
round-robin selection would cause a new database connection to be
established each time for each new worker even when the pressure on the
pool is low (e.g. with just 1 process doing sequential db queries).
Merge pull request #50 from arjan/worker-stack

Use workers as a stack, not a queue
Don't warn about queue type on 17.0

Closes #47.
We're done supporting R15
make pool related typespecs conform to gen_server
fix supervisor typespecs
Merge pull request #51 from sasa1977/fix/pool-typespecs

Fix/pool typespecs
Move gen:start_ret/0 type definition into poolboy.

Because gen doesn't export the start_ret type, we need to move it
into poolboy to actually use it.
Merge pull request #52 from hlieberman/master

Move gen:start_ret/0 type definition into poolboy.
Correctly shutdown the worker supervisor

Closes #53.
Bump version to 1.2.2
Update README
Merge pull request #1 from devinus/master

upadate