Replace mavenCentral() with jcenter() in docs

Fixes gh-1373

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -344,7 +344,7 @@ named ``providedRuntime'', e.g:
 	}
 
 	repositories {
-		mavenCentral()
+		jcenter()
 		maven { url "http://repo.spring.io/libs-snapshot" }
 	}
 

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -182,7 +182,7 @@ Here is a typical `build.gradle` file:
 ----
 	buildscript {
 		repositories {
-			mavenCentral()
+			jcenter()
 ifndef::release[]
 			maven { url "http://repo.spring.io/snapshot" }
 			maven { url "http://repo.spring.io/milestone" }
@@ -202,7 +202,7 @@ endif::release[]
 	}
 
 	repositories {
-		mavenCentral()
+		jcenter()
 ifndef::release[]
 		maven { url "http://repo.spring.io/snapshot" }
 		maven { url "http://repo.spring.io/milestone" }

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -1466,7 +1466,7 @@ To configure IntelliJ correctly you can use the `idea` Gradle plugin:
 [source,groovy,indent=0,subs="verbatim,attributes"]
 ----
 	buildscript {
-		repositories { mavenCentral() }
+		repositories { jcenter() }
 		dependencies {
 			classpath "org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}"
 			classpath 'org.springframework:springloaded:1.2.0.RELEASE'

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -140,7 +140,7 @@ Maven, there is no ``super parent'' to import to share some configuration.
 ----
 	apply plugin: 'java'
 
-	repositories { mavenCentral() }
+	repositories { jcenter() }
 	dependencies {
 		compile("org.springframework.boot:spring-boot-starter-web:{spring-boot-version}")
 	}
@@ -155,7 +155,7 @@ for ``blessed'' dependencies>>:
 [source,groovy,indent=0,subs="attributes"]
 ----
 	buildscript {
-		repositories { mavenCentral() }
+		repositories { jcenter() }
 		dependencies {
 			classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}")
 		}
@@ -164,7 +164,7 @@ for ``blessed'' dependencies>>:
 	apply plugin: 'java'
 	apply plugin: 'spring-boot'
 
-	repositories { mavenCentral() }
+	repositories { jcenter() }
 	dependencies {
 		compile("org.springframework.boot:spring-boot-starter-web")
 		testCompile("org.springframework.boot:spring-boot-starter-test")