Why are there missing peer dependencies after a fresh install?

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [ ] feature request
* [x] question about the decisions made in the repository

### Issue description

**Current behavior:**

`ngx-admin` currently uses angular 5, yet some included packages require angular 4, as can be shown from these npm warnings after doing npm install:
```
npm WARN angular2-toaster@4.0.1 requires a peer of @angular/common@^4.0.0 but none was installed.
npm WARN angular2-toaster@4.0.1 requires a peer of @angular/compiler@^4.0.0 but none was installed.
npm WARN angular2-toaster@4.0.1 requires a peer of @angular/core@^4.0.0 but none was installed.
npm WARN bootstrap@4.0.0-beta.2 requires a peer of jquery@1.9.1 - 3 but none was installed.
npm WARN bootstrap@4.0.0-beta.2 requires a peer of popper.js@^1.12.3 but none was installed.
npm WARN ng2-smart-table@1.2.2 requires a peer of @angular/common@^4.0.0 but none was installed.
npm WARN ng2-smart-table@1.2.2 requires a peer of @angular/core@^4.0.0 but none was installed.
npm WARN ng2-smart-table@1.2.2 requires a peer of @angular/forms@^4.0.0 but none was installed.
npm WARN ngx-echarts@2.0.0 requires a peer of echarts@>=3.1.1 but none was installed.
```
How should these dependencies be resolved?

**Expected behavior:**

I would expect `ngx-admin` not to contain missing peer dependencies within its own selection of packages. Is it that the functionality provided by `ngx-admin` is not affected by the mismatch?

**Steps to reproduce:**

```
git clone https://github.com/akveo/ngx-admin.git
npm install
npm install
```
I did not get any warning after the first `npm install`. So installing a new package or just running a second time `npm install` was required to reproduce the issue.

### Other information:

**npm, node, OS, Browser**
```
node --version
v8.3.0
npm --version
5.3.0
```
On Windows 7.