Upgrade CLI to build 1718

Per dotnet/cli#1783, this should resolve PowerShell/PowerShell#638.

Unfortunately, per dotnet/cli#1839, the Ubuntu package feed is
out-of-date. So now Travis and AppVeyor aren't quite in sync. This is
not great, but the Ubuntu version of CLI does not have the same
regression that the Windows version had.

The readme has been updated to note how to install an actually
up-to-date copy on Linux.

README.md

@@ -76,7 +76,7 @@ non-Windows platforms). Install `dotnet` by following their [documentation][].
 
 The version of .NET CLI is very important, you want a recent 1.0.0 beta
 (**not** 1.0.1). The following instructions will install precisely
-1.0.0.001638, though any 1.0.0 version *should* work.
+1.0.0.001718, though any 1.0.0 version *should* work.
 
 > Previous installations of DNX, `dnvm`, or older installations of .NET CLI
 > can cause odd failures when running. Please check your version.
@@ -89,17 +89,28 @@ The version of .NET CLI is very important, you want a recent 1.0.0 beta
 
 Tested on Ubuntu 14.04.
 
+
+This installs the .NET CLI package feed. The benefit to this is that
+installing `dotnet` using `apt-get` will also install all of its
+dependencies automatically.
+
 ```sh
 sudo sh -c 'echo "deb [arch=amd64] http://apt-mo.trafficmanager.net/repos/dotnet/ trusty main" > /etc/apt/sources.list.d/dotnetdev.list'
 sudo apt-key adv --keyserver apt-mo.trafficmanager.net --recv-keys 417A0893
 sudo apt-get update
-sudo apt-get install dotnet=1.0.0.001638-1
+sudo apt-get install dotnet=1.0.0.001675-1
 ```
 
-Then install the following additional build / debug tools:
+The drawback of using the feed is that it gets out of date. The pinned
+version is the most recent package published to the feed, but newer
+packages are available. To upgrade the package, install it by
+hand. Unfortunately, `dpkg` does not handle dependency resolution, so
+it is recommended to first install the older version from the feed,
+and then upgrade it.
 
 ```sh
-sudo apt-get install g++ cmake make lldb-3.6 strace
+wget https://dotnetcli.blob.core.windows.net/dotnet/beta/Installers/Latest/dotnet-ubuntu-x64.latest.deb
+sudo dpkg -i ./dotnet-ubuntu-x64.latest.deb
 ```
 
 #### OMI
@@ -119,7 +130,7 @@ An MSI installer also exists, but this script avoids touching your system.
 
 ```powershell
 Invoke-WebRequest -Uri https://raw.githubusercontent.com/dotnet/cli/rel/1.0.0/scripts/obtain/install.ps1 -OutFile install.ps1
-./install.ps1 -version 1.0.0.001638 -channel beta
+./install.ps1 -version 1.0.0.001718 -channel beta
 ```
 
 If you meet `Unable to cast COM object of type 'System.__ComObject' to