Upgrade JSCH version

We were sometimes experiencing this issue:

```java
224933 [Connector-16.16.185.16] ERROR com.jediterm.ssh.jsch.JSchTtyConnector  - Error opening session or channel
com.jcraft.jsch.JSchException: channel is not opened.
	at com.jcraft.jsch.Channel.sendChannelOpen(Channel.java:670)
	at com.jcraft.jsch.Channel.connect(Channel.java:151)
	at com.jcraft.jsch.Channel.connect(Channel.java:145)
	at com.jediterm.ssh.jsch.JSchTtyConnector.init(JSchTtyConnector.java:83)
	at com.jediterm.terminal.ui.JediTermWidget$EmulatorTask.run(JediTermWidget.java:224)
	at java.lang.Thread.run(Thread.java:662)
```

which was a known bug of JSCH (see http://hg.netbeans.org/cnd-main/rev/796a74f7f051 for example).

JSCH 0.1.51 is probably fixing it ([changelog](http://www.jcraft.com/jsch/ChangeLog)):

> - bugfix: NPE caused by the delayed response for channel opening requests. FIXED.