Use HTTPS instead of HTTP to resolve dependencies (#675)

This fixes a security vulnerability in this project where the `pom.xml`
files were configuring Maven to resolve dependencies over HTTP instead of
HTTPS.

Signed-off-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>

dubbo-spring-boot-parent/pom.xml

@@ -125,15 +125,15 @@
         <repository>
             <id>spring-milestone</id>
             <name>Spring Milestone</name>
-            <url>http://repo.spring.io/milestone</url>
+            <url>https://repo.spring.io/milestone</url>
             <snapshots>
                 <enabled>false</enabled>
             </snapshots>
         </repository>
         <repository>
             <id>spring-snapshot</id>
             <name>Spring Snapshot</name>
-            <url>http://repo.spring.io/snapshot</url>
+            <url>https://repo.spring.io/snapshot</url>
             <snapshots>
                 <enabled>true</enabled>
             </snapshots>
@@ -159,15 +159,15 @@
         <pluginRepository>
             <id>spring-milestone</id>
             <name>Spring Milestone</name>
-            <url>http://repo.spring.io/milestone</url>
+            <url>https://repo.spring.io/milestone</url>
             <snapshots>
                 <enabled>false</enabled>
             </snapshots>
         </pluginRepository>
         <pluginRepository>
             <id>spring-snapshot</id>
             <name>Spring Snapshot</name>
-            <url>http://repo.spring.io/snapshot</url>
+            <url>https://repo.spring.io/snapshot</url>
             <snapshots>
                 <enabled>true</enabled>
             </snapshots>