twitter-oss: Update to Scala 2.11.12 and 2.12.6 in SBT build

Problem/Solution

We should update our SBT builds to Scala 2.11.12 and 2.12.6.

Additionally, the Slack credentials for each project are out-of-date
and need to be updated.

JIRA Issues: CSL-6972, CSL-6975

Differential Revision: https://phabricator.twitter.biz/D216930

.travis.yml

@@ -19,15 +19,15 @@ before_cache:
   - find $HOME/.sbt -name "*.lock" -delete
 
 scala:
-  - 2.11.11
-  - 2.12.4
+  - 2.11.12
+  - 2.12.6
 
 jdk:
   - oraclejdk8
 
 notifications:
   slack:
-    secure: E0knpocRWQT1YI4TgAZGxS7Xoe15wWZ5fbJsi0nfMhxHSlngrnxAknN60qovPa2J8R/opryX/0jwaUF2/5RPYVxf6ARNI428WkrltxctQeCugN8j5XqpBqXLR/+GPetqFe9WwF6ycg3bmThsXnap6/xNB75zMWj+2E1CYTqwpA0=
+    secure: BFX2aB0sq5cMxmxlWIwgHwJOjI5W6WBn3YSPqHYYz6VAK6fzlX2VUKsfyQJTUnqMYox/0qGzPUB4TFhVMH3c78ocNZNj/9tPOPLkMfHjAqobnq/89+hp4Q6fmWCMKVAGTzVNq83OCykYkzITcYWxo0Pyc2eguW6tHaHIgqnbCRY=
 
 before_script:
   # default $SBT_OPTS is irrelevant to sbt lancher

build.sbt

@@ -23,7 +23,7 @@ val slf4jSimple = "org.slf4j" % "slf4j-simple" % slf4jVersion
 
 val defaultProjectSettings = Seq(
   scalaVersion := "2.12.6",
-  crossScalaVersions := Seq("2.11.11", "2.12.6")
+  crossScalaVersions := Seq("2.11.12", "2.12.6")
 )
 
 val baseSettings = Seq(
@@ -299,7 +299,7 @@ lazy val utilIntellij = Project(
   baseSettings
 ).settings(
   name := "util-intellij",
-  scalaVersion := "2.11.11"
+  scalaVersion := "2.11.12"
 ).dependsOn(utilCore % "test")
 
 lazy val utilJvm = Project(