Edit-user-profile works*

With an awfully big asterisk.

* We don't yet have a way for users/feature specs to access the edit
action without directly entering the URL. Obviously needs fixing;
* UsersController#update is in *dire* need of a DSO;
* config/routes.rb probably needs a cleanup about now;
* Once redcarpet's master branch is usable again, we should track that
instead; which leads to…
* Travis integration would be a Good Thing, including for tracking
either-or cases like redcarpet and rubocop;
* I still think that the `let`s in users_controller_spec could be DRYed
up;
* Coverage is missing a line; will fix Really Soon Now.

RSpec: 537 examples, 0 failures
RuboCop: 134 files inspected, no offenses detected
New UserUpdater DSO for UsersController#update.

The new DSO, given a User entity and a hash of all-optional new
attributes to set on it, modifies the entity and returns a copy of the
(updated) hash of modifiable attributes from `#execute`. Why not the
entity itself? Rails (4.x, anyway) really wants us to call
`#update_attributes` to do that; `#save` is to be used for a new
record. Oh, well.

Next up: adding an edit button to the profile-view page and finally
closing Issue #61.

Other changes:
* The User CCO now knows about the `:slug` attribute;
* Minor version updates in Gemfile.lock;
* UsersControllerSpec has specs for `#update` fully(?) fleshed out;
* User CCO spec knows about the slug; creates instead of building
implementation objects to trigger slug creation.

RSpec: 537 examples, 0 failures
RuboCop: 134 files inspected, no offenses detected
A funny thing happened on the way to the button...

(…with apologies to Steven Sondheim)

While poking around, deciding how best to update the various helpers
involved in building the show-user-record page so that the *owner* of
said record can edit his profile, I was distracted by the fact that
Issue #69 is still open (and will remain so for some time, apparently).
More to the point, fixing `UsersHelper#profile_bio_row` and thus
replacing lines 9-11 of the current `show` template is stymied by
Rails' "helper"'s unhelpful escaping of markup concatenated as a
string.

I did, however, manage to get  the "member can view own profile page
spec" to prove that we're seeing the (HTML) markup we expect, rather
than just the untagged text. Once I quit futzing around with Rails'
`assert_select` and just used Nokogiri.

RSpec: 561 examples, 0 failures
RuboCop: 136 files inspected, no offenses detected
[Fix #61] User-editing UI now accessible.

It's not the solution we wanted, but it's the solution we have. At
least until we spin up a WebSockets implementation for 1.0.  Any user
can visit his own profile page and from there, she and *only* she can
edit her profile data including name, email address and user
bio/profile content (as Markdown).

* Added `app/assets/javascripts/users.js.coffee` to make button element
(as opposed to link) work;
* Added `#profile_bio_header` method to `UsersHelper`;
* Added new `ProfileBioHeaderBuilder` class to support the above;
* Replaced the "raw" header element in app/views/users/show.slim with a
call to `#profile_bio_header`;
* New feature specs to verify correct button visibility for
  * guest user;
  * registered user other than profile owner;
  * profile owner.

RSpec: 571 examples, 0 failures
RuboCop: 139 files inspected, no offenses detected
Merge pull request #79 from jdickey/issue-61-v2

[Fix #61] Merge `issue-61-v2` branch.