Connect Timed Out Error downloading http://downloads.arduino.cc/packages/package_index.json

Hi, I installed the IDE the first time, and am seeing the following error messages while trying to get into the Boards Manager. I can access the URL in my browser with no problem. I am using ubuntu 14.04, and both arduino-1.8.0 and -1.6.13 give me the same error messages.

```
-----------------------------
Error downloading http://downloads.arduino.cc/packages/package_index.json
java.lang.RuntimeException: java.lang.Exception: Error downloading http://downloads.arduino.cc/packages/package_index.json
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onUpdatePressed$0(ContributionManagerUI.java:151)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.Exception: Error downloading http://downloads.arduino.cc/packages/package_index.json
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:131)
at cc.arduino.contributions.packages.ContributionInstaller.download(ContributionInstaller.java:334)
at cc.arduino.contributions.packages.ContributionInstaller.downloadIndexAndSignature(ContributionInstaller.java:309)
at cc.arduino.contributions.packages.ContributionInstaller.updateIndex(ContributionInstaller.java:287)
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onUpdatePressed$0(ContributionManagerUI.java:147)
... 1 more
Caused by: java.net.SocketTimeoutException: connect timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
at sun.net.www.http.HttpClient.(HttpClient.java:211)
at sun.net.www.http.HttpClient.New(HttpClient.java:308)
at sun.net.www.http.HttpClient.New(HttpClient.java:326)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1202)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1181)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1032)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:966)
at cc.arduino.utils.network.FileDownloader.downloadFile(FileDownloader.java:170)
at cc.arduino.utils.network.FileDownloader.download(FileDownloader.java:128)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:129)
... 5 more
```

Start by reproducing the Boards Manager update on Ubuntu 14.04 with the package_index.json URL from the report. Read cc.arduino.utils.network.FileDownloader at FileDownloader.java:170 and the surrounding ContributionInstaller download path; done means the index downloads successfully without the reported connection timeout.

Written by the indexing model from the issue text.