Fix mix.exs deps definition in Mix/OTP Ch. 9

We were referencing the :pipe application as :pipes in the deps
function.
Merge pull request #274 from ernie/fix-mix-deps-in-ch9

Fix mix.exs deps definition in Mix/OTP Ch. 9
Improve chapters based on Ernie's feedback
Mention releases
Touchups to Mix and OTP chapter 1

Minor fixes, including more prominently calling out what I perceive as
an important note regarding dependency environments.
Merge pull request #275 from ernie/chapter-1-touchup

Touchups to Mix and OTP chapter 1
Fixes typos in getting-started/mix/9: pipelines.
Merge pull request #276 from christhekeele/pipeline-typo

Fixes typos in getting-started/mix/9: pipelines.
Replace set_elem with put_elem
typo fix: s/alreay/already/
simplifying sentence structure
simplifying sentence structure
Merge pull request #278 from nicholasf/master

Doc tweaks
replace handle_call/3 with handle_cast/2
Merge pull request #279 from alexfreska/master

replace handle_call/3 with handle_cast/2
Silghtly rewrite wiki wording
Fix copy/paste
Merge pull request #280 from Nami-Doc/patch-1

Fix copy/paste
Minor touchups to wording in OTP chapter 2
Merge pull request #281 from ernie/otp-chapter-2-touchups

Minor touchups to wording in OTP chapter 2