pom.xml: set encoding of Maven to UTF-8

I was getting

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.0.2:testCompile (default-testCompile) on project couchdb-lucene: Compilation failure: Compilation failure:
[ERROR] /home/dyu/src/couchdb-lucene/src/test/java/com/github/rnewson/couchdb/lucene/CustomQueryParserTest.java:[103,40] error: unmappable character for encoding ASCII
[ERROR] /home/dyu/src/couchdb-lucene/src/test/java/com/github/rnewson/couchdb/lucene/CustomQueryParserTest.java:[103,41] error: unmappable character for encoding ASCII
```