Bump the version. (#429)

flexbox-layout has been considered as stable for a while, but from this
on, it starts to follow the semantic versioning.

README.md

@@ -10,7 +10,7 @@ Add the following dependency to your `build.gradle` file:
 
 ```
 dependencies {
-    implementation 'com.google.android:flexbox:0.3.2'
+    implementation 'com.google.android:flexbox:1.0.0'
 }
 ```
 

flexbox/constants.gradle

@@ -22,7 +22,7 @@ ext {
 
     mavenGroup = 'com.google.android'
     mavenArtifactId = 'flexbox'
-    mavenVersion = '0.3.2'
+    mavenVersion = '1.0.0'
 
     bintrayOrg = 'google'
 }