Fix ConcurrentModificationException that occurs when system properties are being modified during a call to ConfigImpl#loadSystemProperties

Occasionally, I get a ConcurrentModificationException at application startup:

```java
Caused by: java.lang.ExceptionInInitializerError: null
	at com.typesafe.config.impl.ConfigImpl.systemPropertiesAsConfigObject(ConfigImpl.java:304)
	at com.typesafe.config.impl.ConfigImpl.systemPropertiesAsConfig(ConfigImpl.java:312)
	at com.typesafe.config.impl.ConfigImpl$LoaderCache.getOrElseUpdate(ConfigImpl.java:48)
	at com.typesafe.config.impl.ConfigImpl.computeCachedConfig(ConfigImpl.java:85)
	at com.typesafe.config.ConfigFactory.load(ConfigFactory.java:278)
	... 26 common frames omitted
Caused by: java.util.ConcurrentModificationException: null
	at java.util.Hashtable$Enumerator.next(Hashtable.java:1048)
	at com.typesafe.config.impl.PropertiesParser.fromProperties(PropertiesParser.java:60)
	at com.typesafe.config.impl.Parseable$ParseableProperties.rawParseValue(Parseable.java:655)
	at com.typesafe.config.impl.Parseable$ParseableProperties.rawParseValue(Parseable.java:637)
	at com.typesafe.config.impl.Parseable.parseValue(Parseable.java:171)
	at com.typesafe.config.impl.Parseable.parseValue(Parseable.java:165)
	at com.typesafe.config.impl.Parseable.parse(Parseable.java:204)
	at com.typesafe.config.impl.ConfigImpl.loadSystemProperties(ConfigImpl.java:293)
	at com.typesafe.config.impl.ConfigImpl.access$000(ConfigImpl.java:26)
	at com.typesafe.config.impl.ConfigImpl$SystemPropertiesHolder.<clinit>(ConfigImpl.java:299)
	... 35 common frames omitted
```

After this is thrown, any further attempts to use `ConfigImpl` result in:

```java
Caused by: java.lang.NoClassDefFoundError: Could not initialize class com.typesafe.config.impl.ConfigImpl$SystemPropertiesHolder
	at com.typesafe.config.impl.ConfigImpl.systemPropertiesAsConfigObject(ConfigImpl.java:304)
	at com.typesafe.config.impl.ConfigImpl.systemPropertiesAsConfig(ConfigImpl.java:312)
	at com.typesafe.config.impl.ConfigImpl$LoaderCache.getOrElseUpdate(ConfigImpl.java:48)
	at com.typesafe.config.impl.ConfigImpl.computeCachedConfig(ConfigImpl.java:85)
	at com.typesafe.config.ConfigFactory.load(ConfigFactory.java:278)
```

This patch fixes this issue by copying the system properties in a synchronized block and then using the copy in the call to `Parseable#parse`.