update commons-lang to 3.8.1 (#34)

diff --git a/doxia-core/pom.xml b/doxia-core/pom.xml
index c9ae50a..dee97df 100644
--- a/doxia-core/pom.xml
+++ b/doxia-core/pom.xml
@@ -57,7 +57,7 @@
     <dependency>
       <groupId>org.apache.commons</groupId>
       <artifactId>commons-lang3</artifactId>
-      <version>3.5</version>
+      <version>3.8.1</version>
     </dependency>
     <dependency>
       <groupId>org.apache.httpcomponents</groupId>