Revert Cobertura DTD location change.

Commit 1afa9cc changed Cobertura DTD location from http://cobertura.sourceforge.net/xml/coverage-04.dtd to https://raw.githubusercontent.com/cobertura/cobertura/master/cobertura/src/site/htdocs/xml/coverage-04.dtd because the first resource was not accesible for some period of time. Now it works. On the other hand there are weird problems with the GitHub location (see scoverage/scoverage-maven-plugin#27 and scoverage/sbt-coveralls#80).

Reverting to the previous location. It's [still used](https://github.com/cobertura/cobertura/blob/cobertura-2.1.1/cobertura/src/main/java/net/sourceforge/cobertura/reporting/xml/XMLReport.java#L66-L67) by Cobertura project.

Author: gslowikowski

scalac-scoverage-plugin/src/main/scala/scoverage/report/CoberturaXmlWriter.scala

@@ -17,7 +17,7 @@ class CoberturaXmlWriter(sourceDirectories: Seq[File], outputDir: File) extends
 
   def write(coverage: Coverage): Unit = {
     val file = new File(outputDir, "cobertura.xml")
-    IOUtils.writeToFile(file, "<?xml version=\"1.0\"?>\n<!DOCTYPE coverage SYSTEM \"https://raw.githubusercontent.com/cobertura/cobertura/master/cobertura/src/site/htdocs/xml/coverage-04.dtd\">\n" +
+    IOUtils.writeToFile(file, "<?xml version=\"1.0\"?>\n<!DOCTYPE coverage SYSTEM \"http://cobertura.sourceforge.net/xml/coverage-04.dtd\">\n" +
         new PrettyPrinter(120, 4).format(xml(coverage)))
   }
 

scalac-scoverage-plugin/src/test/resources/scoverage/cobertura.sample.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<!--DOCTYPE coverage SYSTEM "https://raw.githubusercontent.com/cobertura/cobertura/master/cobertura/src/site/htdocs/xml/coverage-04.dtd"-->
+<!DOCTYPE coverage SYSTEM "http://cobertura.sourceforge.net/xml/coverage-04.dtd">
 
 <coverage line-rate="0.9" branch-rate="0.75" version="1.9" timestamp="1187350905008">
     <sources>