Update .travis.yml and some doc comments

.travis.yml

@@ -6,9 +6,9 @@ matrix:
       dist: trusty
       before_install:
         - wget -q -O - https://swift.org/keys/all-keys.asc | gpg --import -
-        - wget https://swift.org/builds/swift-4.0-branch/ubuntu1404/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-13-a/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-13-a-ubuntu14.04.tar.gz
-        - tar xzf swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-13-a-ubuntu14.04.tar.gz
-        - export PATH=${PWD}/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-13-a-ubuntu14.04/usr/bin:"${PATH}"
+        - wget https://swift.org/builds/swift-4.0-branch/ubuntu1404/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-08-04-a/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-08-04-a-ubuntu14.04.tar.gz
+        - tar xzf swift-4.0-DEVELOPMENT-SNAPSHOT-2017-08-04-a-ubuntu14.04.tar.gz
+        - export PATH=${PWD}/swift-4.0-DEVELOPMENT-SNAPSHOT-2017-08-04-a-ubuntu14.04/usr/bin:"${PATH}"
       script:
         - swift test -Xcc -D_GNU_SOURCE=1
     - os: osx

Sources/Math.swift

@@ -5,11 +5,10 @@
 //  Created by Xiaodi Wu on 3/31/17.
 //
 
-/// A signed numeric type that provides square root, cube root, and elementary
-/// transcendental functions.
+/// A signed numeric type that supports elementary functions.
 ///
 /// The `Math` protocol provides a suitable basis for writing functions that
-/// work on any real or complex floating-point type which provides the required
+/// work on any real or complex floating-point type that supports the required
 /// functions.
 public protocol Math : SignedNumeric {
   /// The mathematical constant pi (_π_).

Sources/Real.swift

@@ -11,10 +11,11 @@ import Glibc
 import Darwin.C
 #endif
 
-/// A floating-point type that provides a selection of special functions.
+/// A floating-point type that supports elementary functions and a selection of
+/// special functions.
 ///
 /// The `Real` protocol provides a suitable basis for writing functions that
-/// work on any floating-point type that provides the required functions.
+/// work on any floating-point type that supports the required functions.
 public protocol Real : Math, FloatingPoint {
   /// Returns the hypotenuse of a right-angle triangle with legs (catheti) of
   /// length `x` and `y`, preventing avoidable arithmetic overflow and