Fix missing apache commons-lang dependency

Used multiple times and no longer included by indirect dependencies,
as it seems.

[ERROR] Failed to execute goal
org.apache.maven.plugins:maven-compiler-plugin:3.12.1:compile
(default-compile) on project Kustvakt: Compilation failure: Compilation
failure:
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/constant/TokenType.java:[3,31]
package org.apache.commons.lang does not exist
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/constant/QueryType.java:[3,31]
package org.apache.commons.lang does not exist
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/constant/AuthenticationScheme.java:[3,31]
package org.apache.commons.lang does not exist
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/encryption/RandomCodeGenerator.java:[13,31]
package org.apache.commons.lang does not exist
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/utils/StringUtils.java:[3,31]
package org.apache.commons.lang does not exist
[ERROR]
/home/kupietz/KorAP/Kustvakt/src/main/java/de/ids_mannheim/korap/encryption/KustvaktEncryption.java:[10,31]
package org.apache.commons.lang does not exist
[ERROR] -> [Help 1]

Change-Id: I987e1ae0e9230b7a14e33cd60896c05d01085494
diff --git a/pom.xml b/pom.xml
index f26a344..d651bbe 100644
--- a/pom.xml
+++ b/pom.xml
@@ -577,6 +577,11 @@
 			<version>2.12.6</version>
 		</dependency>
 		<dependency>
+			<groupId>commons-lang</groupId>
+			<artifactId>commons-lang</artifactId>
+			<version>2.6</version>
+		</dependency>
+		<dependency>
 			<groupId>commons-io</groupId>
 			<artifactId>commons-io</artifactId>
 			<version>2.15.1</version>